The utility model relates to a mold for vulcanizing an underwater connector, and particularly to a mold for manufacturing a watertight connector plug. The mold comprises the components of: an upper mold, a lower mold, a fastening screw, a left mold core between the upper mold and the lower mold, a positioning pin, a jack, a housing and a right mold core. Firstly roughening, cyclizing and bottom glue coating are performed on the jack and the housing. After the bottom glue is dried, the housing, the jack and the positioning pin are arranged in the mold core. Then the mold core is placed in a preheated mold cavity. Then the mold is closed and placed on a vulcanizing machine. Pressurizing, glue injecting and vulcanizing are performed. After a preset time period, a workpiece is cured and then is taken out of the mold. The mold for manufacturing the watertight connector plug is important in the fastening screw; after the upper mold and the lower mold are closed, the fastening screw is fastened so that the left mold core, the jack and the right mold core closely contact with one another; and no clearance exists between the end surface of the jack and the end surface of the positioning pin on the left mold core, thereby ensuring no glue entering into the jack in the glue injection process. The mold is suitable for manufacturing different types of watertight connector plugs.